Clock and data recovery (CDR) in digital signal processing (DSP) based optical receivers is challenging if the received signal is distorted by chromatic dispersion, polarization mode dispersion or non-linear effects. Traditional digital clock recovery schemes are too complex to be implemented at data rates typically used in optical communications.
Clock and data recovery in digital signal processing (DSP) based optical receivers can be performed either in analog domain, digital domain, or in a mixed-mode fashion.
In an analog clock and data recovery scheme, the unprocessed signal before digitization is used for extracting a clock signal. This imposes limits on the maximum signal distortion (chromatic dispersion, polarization-mode dispersion, non-linearity) a receiver can handle independently of post-compensation.
In a digital clock and data recovery scheme, the digitized signal is processed by (1) extracting the actual clock signal, and (2) re-processing the sampled data so that it reflects the timing estimate extracted in the clock recovery scheme. Some clock signal extraction techniques in the digital domain are based on Fast Fourier Transform (FFT)-based tone extraction, while data recovery is typically performed using an interpolation filter with adjustable coefficients or a using a bank of interpolation filters with fixed coefficients. The adjustment of the coefficients in the first case or the selection of the output filter in the second case is based on the phase of the recovered clock.
In a mixed-mode clock and data recovery scheme, a control signal is fed back to an oscillator that creates a clock signal for an analog-to digital conversion based on the digitally recovered clock.
The non-data aided (NDA) symbol timing recovery method known today as Gardner timing error detector and other methods described in Digital Communication Receivers, H. Meyr, Jon Wiley & Sons, incorporated herein by reference in its entirety, that are developed for DSL and wireless communications, can be useful for non-PDM systems or PDM systems with slowly varying polarization, but they are inadequate to cope with PDM systems in general, especially with rapidly changing polarization and polarization mode dispersion of the optical transport system. Also, conventional methods suffer from chromatic dispersion. Prior approaches may also be limited by jitter tolerance and polarization tracking speed due to the feedback loop used in the method.